This invention relates to current collector electrode supports for fuel cells. More specifically, this invention relates to corrugated current collectors for high temperature, internally reforming, planar fuel cells.
A fuel cell is a device which directly converts chemical energy stored in a fuel such as hydrogen or methane into electrical energy by means of an electrochemical reaction. This differs from traditional electric power generating methods which must first combust the fuel to produce heat and then convert the heat into mechanical energy and finally into electricity. The more direct conversion process employed by a fuel cell has significant advantages over traditional means in both increased efficiency and reduced pollutant emissions.
In general, a fuel cell, similar to a battery, comprises a negative (anode) electrode and a positive (cathode) electrode separated by an electrolyte which serves to conduct electrically charged ions between them. In contrast to a battery, however, a fuel cell will continue to produce electric power as long as fuel and oxidant are supplied to the anode and cathode, respectively. To achieve this, gas flow fields are provided adjacent to the anode and cathode through which fuel and oxidant are supplied. In order to produce a useful power level, a number of individual fuel cells must be stacked in series with an electrically conductive separator plate between each cell.
In internally reforming fuel cells, a steam reforming catalyst is placed within the stack of fuel cells to allow direct use of hydrocarbon fuels (e.g., methane, coal gas, etc.) in the fuel cells without the need for expensive and complex external reforming equipment. Two forms of internal reforming have been used. Indirect internal reforming is accomplished by placing reforming catalyst in an isolated chamber within the stack and routing the reformed gas from this chamber into the anode compartment of each fuel cell. Direct internal reforming is accomplished by placing reforming catalyst within the active anode compartment of each fuel cell. This catalyst is then available to reform fuel gas with steam formed by the electrochemical reactions of the fuel cell and can result in very high reforming efficiency and fuel utilization.
The particular geometry used for the anode chamber and catalyst support is important for many reasons. The geometry should be selected to minimize pressure drop for fuel flow as higher pressure requires more auxiliary power and results in lower system efficiency. In addition, for stacks with many cells, low variation in flow restriction from cell to cell is critical to insure that each cell receives the same fuel flow and thereby operates at the same fuel utilization. As the efficiency of the fuel cell stack is limited by the individual cell which receives the least amount of fuel, uniform fuel flow is very important to achieve maximum fuel utilization and stack electrical efficiency.
A major feature of a fuel cell which also relates to direct internal reforming catalyst is the useful life of the cell. As the cell ages, the activity of the reforming catalyst in the anode chamber decays due to the poisoning effects of the electrolyte vapor within the passage. This decay in activity reduces the effectiveness of the catalyst and the reforming efficiency of the cell. This, in turn, reduces the electrical efficiency of the cell because less reformed fuel is available for the electrochemical reactions. As the effectiveness of the catalyst is directly related to the mass of catalyst available for reforming, one way to increase the reforming efficiency and cell life is to increase the catalyst mass in the anode chamber.
Another important characteristic of the components used to form the fuel and oxidant flow fields of a fuel cell is the ability of the components to apply uniform pressure to the active cell components (i.e., anode,.cathode and matrix). Uniform pressure is important to insure uniform contact resistance over the cell active area as well as to reduce the likelihood of gaps forming between cell components during operation.
The direct internal reforming catalyst must also be protected from deactivation by electrolyte wicking from the abutting anode containing liquid electrolyte. One method to protect the catalyst is to design the corrugated current collector so that it shields the catalyst, acting as a barrier to electrolyte migration from the adjacent anode component.
Many different component geometries have been proposed and used by fuel cell manufacturers for providing the fuel and oxidant flow fields and catalyst support for direct internally reforming fuel cells. U.S. Pat. No. 4,548,876 describes a corrugated current collector used for this purpose. In the current collector of the ""876 patent, particulate material is placed in the current collector corrugations and is used for diffusion and support. The particulate material faces a catalyst layer which, in turn, abuts an active electrode. The particulate material is preferably made of nonconducting alumina, but may also comprise the same material as used in the catalyst layer. This geometry is limited in that a considerable portion of the active electrode is blocked by the particulate material and the gas is forced to flow around the blockage away from the electrode.
U.S. Pat. No. 4,983,472 describes a corrugated current collector with a plurality of corrugations forming dimples arranged in a checkerboard pattern for use on the cathode side of the cell. This configuration represents an improvement over the current collector of the ""876 patent by allowing the gas much better access to the active electrode. However, the configuration has limitations when used in an anode chamber which also houses a direct internal reforming catalyst in the form of elongated solid elements. Due to the checkerboard pattern, these catalyst elements can only be loaded substantially perpendicular to the fuel gas flow. This results in high flow restriction and susceptibility to large variation in flow restriction from cell to cell.
Another disadvantage of the checkerboard pattern of dimples used in the current collector of the ""472 patent is that when applied to both the anode and cathode chambers, a larger scale checkerboard pattern of compressive load results on the cell active area due to the periodic nesting of the corrugation feet through the bipolar plate. This non-uniform pressuredistribution could result in variation in the cell contact resistance.
A further concern when using the current collector of the ""472 patent relates to the flow field formed by placing the solid catalyst elements perpendicular to the flow direction. The resulting geometry is characterized by nearly equal resistance to flow parallel and perpendicular to the primary flow direction. This means that as gas is generated in high current producing areas of the cell it is allowed to expand laterally, apparently enhancing mixing within the cell. However, this also tends to supply high current producing areas of the cell with even more fresh fuel resulting in high current density and temperature gradients within the cell.
U.S. Pat. No. 5,795,665 describes an alternate corrugated current collector design wherein the current collector is combined with a separator plate to form gas passages and support for solid catalyst elements. In this design, all the cell components are formed in a corrugated pattern and nested together. As a result forming the cell components is a complex operation. Also, the space available for the reforming catalyst elements is limited.
It is therefore an object of the present invention to provide a novel corrugated current collector design which overcomes the disadvantages of the prior art designs.
It is also an object of the present invention to provide a corrugated current collector which allows solid direct internal reforming catalyst elements to be loaded substantially parallel to the primary direction of gas flow.
It is a further object of the present invention to provide a corrugated current collector which allows the space provided for solid reforming catalyst elements to be maximized.
It is yet a further object of the present invention to provide a corrugated current collector which results in more uniform pressure distribution to the cell active components.
It is still a further object of the present invention to provide a corrugated current collector which results in a pattern flow field geometry characterized by high transverse flow resistance and low axial flow resistance thus resulting in more uniform current and temperature distributions within the cell.
It is also an object of the present invention to provide a corrugated current collector which results in better shielding of the direct internal reforming solid catalyst elements from the cell electrolyte and holding these catalyst elements away from the electrolyte containing electrode.
In accordance with the principles of the present invention, the above and other objectives are realized in a current collector having successive spaced rows of corrugations, with the corrugations in a given row establishing successive peak and valley regions along the given row and the spaced rows of corrugations being adapted so that corresponding peak regions from row-to-row establish through passages for receiving and supporting solid catalyst elements.
In the embodiment of the invention to be described hereinafter, the spaced rows of corrugations are such that there is a given pitch between successive peak regions in a row and a given offset in the peak regions from row-to-row. In particular, the offset is selected to be finite and to be less than 50 percent of the pitch, so as to establish the plurality of through passages for the solid catalyst elements. Also, the offset is further selected based on the catalyst dimensions and is such that the corrugations engage the catalyst elements.